Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $74.40   3.837%   $77.2547   $77.25   $77.25   $77.30  
  Purchase #2 $109.45   5.228%   $115.172   $115.17   $115.15   $115.20  
  Purchase #3 $199.55   10.444%   $220.391   $220.39   $220.40   $220.40  
  Purchase #4 $71.62   5.167%   $75.3206   $75.32   $75.30   $75.30  
  Purchase #5 $78.46   12.304%   $88.1137   $88.11   $88.10   $88.10  
  Purchase #6 $19.79   6.793%   $21.1343   $21.13   $21.15   $21.10  
  Purchase #7 $75.84   7.454%   $81.4931   $81.49   $81.50   $81.50  
  7 purchase totals = $629.11       $678.8794   $678.86   $678.85   $678.90